More recently, glasses are being increasingly used in ophthalmic optics, which glasses are distinguished by a high refractive index and low density. By virtue of these glasses, thinner and lighter lenses can be produced, which is particularly advantageous to eyeglass wearers having a need for greater corrections of defective vision. For high-refractive index glasses (see, e.g., U.S. Pat. No. 4,084,978), primarily colorless or ionically tinted types have been provided. Since phototropic glasses have gained comparatively greater notoriety than corrective spectacle lenses in recent years, there is thus a demand for higher-refractive index phototropic glasses.
The first phototropic glass possessing the desired properties of phototropism, a high refractive index and low density was achieved according to EP 0 063 790-A. But these glasses have the drawback that they must contain a relatively high halide portion in order to satisfy the desirable kinetics of the phototropism, i.e., sufficiently quick darkening in the case of insolation and brightening in the case of deficient exposure. But, because of high and varying evaporation of the halides when the glass batch is melted, a constancy of the phototropic properties can be maintained only with a great deal of difficulty.
Conversely, a high-refractive index phototropic glass with low density and low halogen contents was described for the first time in DE 38 25 210-C2, but this glass has a refractive index n.sub.d of only between about 1.60 and 1.61, an Abbe coefficient between about 41.5 and 43, and a density of less than 3 g/cm.sup.3.